It’s is a good idea, before entering the water, to learn a little very elementary physics that may affect your scuba diving experience.
No, as a scuba diver you don’t have to be scientist; even through you will find a lot of scuba diving scientists. But if don’t want your first scuba diving experience to be your last, you should gain a little knowledge before you dive to make it a safe dive.
Buoyancy and Archimedes Principle
You learn from childhood that some things placed in water float, others tend to sink to the bottom. Whether you float or sink when you enter the water depends on the buoyancy of your body.
What is Archimedes’ Principle?
Archimedes' Principle states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by the object. So, what does that means?
It means that is not only the weight of the object that count, an example is a loaded aircraft carrier can weigh up to 100,000 pounds but it can still float! The 100,000 loaded aircraft carrier can float because it displaces enough water so that the force pushing up from underneath the carrier balances the force of the gravity acting down.
The Archimedes’ Principle applies the same to people whether they are in fresh water or salt water.
For most beginners’ scuba divers, here is a physical idea that they tend to misunderstand. If you weigh two hundred pounds out of the water, how much do you weigh in the water?
Most people thinks you weigh less in water buy you really weight the same whether you are in water or out of water, so the answer is two hundred pounds.
Your weight is the product of how much mass, m, you have multiplied by the acceleration due to gravity, g. W = mg. You don't have less mass in water and g (near the Earth) is about 32 ft/s^2 or 9.8 m/s^2 in or out of the water. All this means is you weigh the same in water as out of water.
What's changed in water is that there is an upward directed buoyant force produced by the water underneath you. The water on top pushes downward. The balance of these forces, which is greater, determines whether you'll sink or float.
You have positive buoyancy if you float, negatively buoyancy if you sink, and you have neutral buoyancy if you are stationary.
The air in your lungs tends to make you positively since air is less dense than water. The weight of you body counteracts the upward force, the buoyancy compensator and the other gear you carry.
The numbers depend not only your specific weight and that of the gear but also on the kind of water that you dive in. Because salt water is denser and therefore heavier, you have to displace less of it in order to be positively buoyant.
AirPressure
How much does air weigh?
A column of air as tall as the atmosphere would weigh about 14.7 lbs (6.7 kg). Thus, the air pressure per square inch acting on your head and shoulders is about 14.7 lbs per square inch.
Water exerts a much greater force on you because it is much denser and this, of course, makes it heavier. It only takes a thirty-three foot high column of water to weigh 14.7 lbs.
Thus the water pressure at a depth of 33 feet is 2 atm. (1 atm from the air + 1 atm from the water. atm = atmosphere, a unit of pressure.)
Common sense says the deeper you go, the greater the pressure. Now you know by how much. At 66 feet you experience 3 atm, at 99 feet 4 atm, and so forth.
Gauge pressure - that shown on a depth gauge - will discount the 1 atm from the atmosphere.